Ion implantation is a standard technique for introducing conductivity—altering impurities into a workpiece such as a semiconductor wafer. A desired impurity material may be ionized in an ion source, the ions may be accelerated to form an ion beam of prescribed energy, and the ion beam may be directed at a front surface of the wafer. The energetic ions in the beam penetrate into the bulk of the semiconductor material and are embedded into the crystalline lattice of the semiconductor material to form a region of desired conductivity. The ion beam may be distributed over the wafer area by beam movement, wafer movement, or by any combination thereof.
Traditionally, ion implanters strive to introduce the impurities at a uniform dose across the front surface of the wafer and lack the ability to provide a non-uniform dose pattern. Dose is usually measured in a number of implanted ions per unit area. However, there have been recent trends towards providing a non-uniform dose pattern. A non-uniform dose pattern may be utilized to mitigate the effect of spatial variation in one or more other process steps during semiconductor device manufacturing. For example, the impact of non-uniform etch patterns may be at least partially compensated for by altering the implant dose with an inverse spatial variation.
To create a desired non-uniform dose pattern, one conventional ion implanter may utilize a hard mask to block the ion beam in specified areas from striking the wafer. Other conventional ion implanters may limit wafer movement and/or beam scanning to effectively limit the ion beam to specified areas. Multiple passes of the ion beam relative to the front surface of the wafer can then create uniform but different doses in different regions. Drawbacks of these approaches include the additional time for multiple passes which adversely affects throughput performance. In addition, the actual implanted non-uniform dose pattern may not accurately match the desired non-uniform dose pattern since the desired dose pattern can only be approximated with multiple passes of different uniform doses in different regions.
Another conventional ion implanter having a scanned ion beam creates a non-uniform dose pattern by defining and controlling the scan velocity of the scanned ion beam in one scanned direction. However, the defined scan velocity of the beam in the scanned direction is not modified as the wafer is driven in a direction orthogonal to the scanned direction. Accordingly, one pass of the wafer through the scanned beam is made, the wafer is rotated, and an additional pass is made, and so on. A drawback of this approach is that it requires multiple passes of the wafer relative to the ion beam which adversely affects throughput performance. It also requires the wafer to be rotated before each pass and the resulting dose pattern is largely limited to symmetrical patterns.
Accordingly, there is a need for new and improved methods and apparatus for directly creating a desired two-dimensional non-uniform dose pattern.